The present invention describes materials and methods for achieving alignment of liquid crystal materials on a substrate surface and devices fabricated using these methods and materials.
Liquid crystal display devices (LCDs) or light shutters generally comprise a layer of liquid crystalline material between two solid substrates to form a cell. These substrates are generally coated with a conducting material, such as Indium/Tin Oxide (ITO) to form electrodes or electrode patterns. An electric field applied across the cell or between the electrodes switches the liquid crystal between different molecular arrangements or states. Thus the light transmission through the cell can be modulated depending on the cell configuration, the type of liquid crystalline material, the presence of polarisers, etc. A preferred molecular alignment direction and pretilt angle (xcex8) is imparted by an alignment layer on top of the electrodes and in contact with the liquid crystalline material.
It is well known in the art that fabrication of liquid crystal devices which have advantageous performance and low defect densities requires control of the alignment of the liquid crystalline material at the surfaces of the device. Different types of liquid crystal alignment have been described. Homeotropic alignment refers to an alignment in which the unique optical axis of a liquid crystal phase is held perpendicular to the adjacent surface.
Planar alignment, sometimes referred to as homogeneous alignment, refers to alignment in which the unique optic axis of the liquid crystal phase lies parallel to the adjacent surface. Planar alignment may also impose a direction in which the optic axis of the liquid crystal lies in the plane of the adjacent surface.
Tilted planar alignment or tilted homogeneous alignment refer to alignment in which the liquid crystal unique optic axis lies at an angle, termed the pretilt angle (xcex8) from the plane of the adjacent surface. The pretilt angle may be as small as a fraction of one degree or as large as several tens of degrees.
Tilted homeotropic alignment refers to an alignment in which the optic axis of the liquid crystal lies tilted away from the normal to the adjacent surface. This deviation is again termed a pretilt angle.
In liquid crystal devices, said alignment geometries are chosen and used in combination to achieve specific optical and electro-optic properties from the device and may be combined in new ways or with new liquid crystalline mixtures to provide new types of devices.
Several methods are known in the art by which defined liquid crystal alignment may be achieved. Deposition of a polymer layer, for example a polyimide layer, on the substrate surface followed by mechanical rubbing provides a pretilted planar alignment. A planar alignment or tilted planar alignment may also be achieved by evaporating a variety of inorganic substances, for example SiOx, onto the surface from an oblique angle of incidence. A disadvantage of this method is that it requires slow and costly vacuum processing. A further disadvantage is that the resulting evaporated layer may show a high capacity to absorb contaminants onto itself from the environment or from other materials used in fabrication of the device.
A homeotropic alignment can be obtained by depositing a surfactant, for example a quaternary ammonium salt, onto the surface from solution in a suitable solvent, a disadvantage of this treatment is that the resistivity of the liquid crystal device may be lowered by the surfactant and the resulting alignment may also show poor stability.
Structured alignment patterns of subpixel size and above can be achieved by illumination of a polymer layer containing photochemically orientable dyes or photochemically dimerisable and/or isomerisable molecules, as described, for example, in EP-A-0445629. A disadvantage of this method is that the solubility of the dye molecules in the polymer matrix is limited and the chemical and photochemical stability over time is insufficient.
Another method for achieving structured non-contact orientation is the photodimerisation of polymers incorporating photodimerisable groups, such as cinnamate or coumarin derivatives, as described, for example, in Jpn. J. Appl. Phys., Vol., 31, 2155 (1995) and EP-A-9410699.0. A disadvantage of these materials is the polydispersity of the materials produced by polymerisation. This requires, for example, different solution concentrations for spin coating depending on the average molecular weights of the polymers which can not be determined with any great accuracy and which are often not reproducible from one batch to another. This can give rise to unreproducible alignment as well as also requiring repeated purification cycles of the polymer product in order to remove unreacted monomer and oligomers. The attachment of low molar mass photoreactive units to monodispersed polymer backbones can lead to polymers with unreacted sites, which can give rise to dielectric breakdown of cells containing such materials. This is especially important for active matrix devices.
An object of this invention is to provide means of achieving a defined surface alignment of a liquid crystalline material on a substrate surface, which does not require mechanical rubbing or other methods of physical contact which may damage the surface or structures on the surface. This is especially important for active matrix displays based on the use of surface mounted thin film transistors. Static electricity or dust caused by mechanical rubbing or buffing polymer layers, such as polyimide or polyamide, in order to induce a unidirectional alignment due to microgrooves can cause defects in thin film transistors and lead to dielectric breakdown. Such alignment layers also suffer from the disadvantage that the microgrooves possess inherent defects themselves, which can result in random phase distortion and light scattering. This impacts detrimentally on the optical appearance of the displays or the efficiency of the light shutters. Additionally, mechanical buffing does not allow locally oriented regions of the surface to be aligned with different azimuthal angles. This is a substantial drawback since sub-pixelisation can lead to higher contrast and an improved optical efficiency.
According to this invention compounds are provided of Formula I: 
wherein
X1-5 are independently selected from H, F, CN, phenylene, C1-10 alkyl whereby when X1=X2, then X1,2=H and when X3=X4, then X3,4=H
S1-3 are spacer units
PG1-3 are photoisomerisable/dimerisable groups.
The term xe2x80x9cspacer unitsxe2x80x9d S1-3 include, for example, independently of one another,
an alkylene unit with 1 to 16, preferably 1 to 10, carbon atoms wherein the alkylene unit may have one or more non-adjacent CH2 groups substituted with COO, OOC, O;
a cycloalkylene group with 3 to 8 carbon atoms, preferably with 5 or 6 carbon atoms, in which optionally one or two methylene units can be replaced by NH groups;
phenylene, which can be unsubstituted or substituted from one and up to and including all available substitution positions with C1-10 alkyl, C1-10 alkoxy, CN, NO2, halogen, or carbonate; 
COO, OOC;
an amide group, that is, xe2x80x94CONHxe2x80x94 and xe2x80x94NHOCxe2x80x94, the H group on the amide may be substituted with C1-10 alkyl groups;
an ether group, that is COC.
Particularly preferred spacer groups for S1-3 include oxycarbonylalkanoyloxy, oxyalkoxy, oxycarbonylalkoxy, oxyalkanoyloxy, oxycarbonylphenoxyalkanoyloxy, oxyalkoxyalkyl containing from 1-16 carbon atoms.
The isomerisable/dimerisable units PG1-3 are molecular units which can undergo either photochemical cis/trans isomerisation and/or photochemical cycloaddition and thus lead to a cross-linking of the molecule. The isomerisation/dimerisation units PG1-3 are linked via the spacer units S1-3 to the propane backbone and can either have the general formula II 
wherein
A may be 1,2-, 1,3- or 1,4-phenylene, which is unsubstituted or substituted from one and up to and including all available substitution positions with one or more of the groups selected from halogen, CN, NO2 and in which 1 or 2 CH groups can be replaced by nitrogen, or A may be 2,5-thiophenediyl, 2,5-furanylene, 1,4- or 2,6-naphthalene in which 1 or 2 CH groups can be replaced by nitrogen;
A1 may be 1,2-, 1,3- or 1,4-phenylene, which is unsubstituted or substituted from one and up to and including all available substitution positions with one or more of the groups selected from halogen, CN, NO2 and in which 1 or 2 CH groups can be replaced by nitrogen, or A1 may be 2,5-thiophenediyl, 2,5-furanylene, 1,4- or 2,6-naphthalene in which 1 or 2 CH groups can be replaced by nitrogen;
A2,3 may each independently be selected from 1,2-, 1,3- or 1,4-phenylene, which is unsubstituted or substituted from one and up to and including all available substitution positions with one or more of the groups selected from halogen, CN, NO2 and in which 1 or 2 CH groups can be replaced by nitrogen;
or A2,3 may each independently be selected from 2,5-thiophenediyl, 2,5-furanytene, 1,4- or 2,6-naphthalene in which 1 or 2 CH groups can be replaced by nitrogen, trans-1,3-dioxane-2,5-diyl or 1,4-piperidyl;
Z1,2 may each independently be selected from a direct bond, CH2CH2, COO, OOC, O(CH2)3, OCH2, CH2O, (CH2)3O, (CH2)4, or the trans form of OCH2CHxe2x95x90CH, CH2CHxe2x95x90CHO, CH2CH2CHxe2x95x90CH, CH2CHxe2x95x90CHCH2;
n1,2 may each independently be 0 or 1;
W, Y may each independently be selected from H, halogen, CN, alkyl with 1 to 12 carbon atoms which is optionally substituted with one or more fluorines and in which optionally 1 or 2 non-adjacent methylene units (CH2) can be replaced by oxygen, COO, OOC, CO and/or CHxe2x95x90CH,
R1 may be H, halogen, CN, NO2, NCS, alkyl with 1 to 12 carbon atoms which is optionally substituted with one or more fluorines and in which optionally 1 or 2 non-adjacent methylene units (CH2) can be replaced by oxygen, COO, OOC, CO and/or CHxe2x95x90CH;
or PG1-3 may have the formula III 
wherein
A4,5 may each independently be selected from 1,2-, 1,3- or 1,4-phenylene, which is unsubstituted or substituted from one and up to and including all available substitution positions with one or more of the groups selected from halogen, CN, NO2 and in which 1 or 2 CH groups can be replaced by nitrogen;
or A4,5 may be independently seiected from 2,5-thiophenediyl, 2,5-furanylene, 1,4- or 2,6-naphthalene in which 1 or 2 CH groups can be replaced by nitrogen;
Z3,4 may each independently be selected from a direct bond, CH2CH2, COO, OOC, O(CH2)3, OCH2, CH2O, (CH2)3O, (CH2)4, or the trans form of OCH2CHxe2x95x90CH, CH2CHxe2x95x90CHO, CH2CH2CHxe2x95x90CH, CH2CHxe2x95x90CHCH2;
n3,4 may each independently be 0 or 1;
A6 may be 
A7 may be 1,2-, 1,3- or 1,4-phenylene, which is unsubstituted or substituted from one and up to and including all available substitution positions with one or more of the groups selected from halogen, CN, NO2 and in which 1 or 2 CH groups can be replaced by nitrogen,
or A7 may be 2,5-thiophenediyl, 2,5-furanylene, 1,4- or 2,6-naphthalene in which 1 or 2 CH groups can be replaced by nitrogen, trans-1,3-dioxane-2,5-diyl or 1 4-piperidyl;
Z5 may be an alkylene unit with 1 to 16, preferably 1 to 10, carbon atoms,
a cycloalkylene group with 3 to 8 carbon atoms, preferably with 5 or 6 carbon atoms, in which optionally one or two methylene units can be replaced by NH groups,
or phenylene, which can be substituted from one and up to and including all available substitution positions with one or more of the groups selected from C1-10 alkyl, C1-10 alkoxy, CN, NO2, halogen, or carbonate, 
an ester group, that is, COO and OOC;
an amide group, that is, xe2x80x94CONHxe2x80x94 and xe2x80x94NHOCxe2x80x94, the H group on the amide may be substituted with C1-10 alkyl groups;
an ether group, that is, COC;
n5 may be 0 or 1;
R2 may be H, halogen, CN, NO2, NCS, SCN, alkyl with 1 to 12 carbon atoms which is optionally substituted with one or more fluorines and in which optionally 1 or 2 non-adjacent methylene units (CH2) can be replaced by oxygen, COO, OOC, CO and/or CHxe2x95x90CH;
R3 may be H, halogen, CN, NO2, NCS, SCN, alkyl with 1 to 12 carbon atoms which is optionally substituted with one or more fluorines and in which optionally 1 or 2 non-adjacent methylene units (CH2) can be replaced by oxygen, COO, OOC, CO and/or CHxe2x95x90CH;
R4 may be H or C1-10 alkyl;
V may be H, alkyl or alkoxy with 1 to 8 carbon atoms, trifluoromethyl, or phenyl which may be substituted from one and up to and including all available substitution positions with one or more of the groups selected from CN, halogen, NO2;
U may be H, alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR5; 
R5 may be C1-10 alkyl;
r may be a number from 0 to 3;
X6,7 may be independently selected from H, halogen, CN, NO2, NCS, SCN, alkyl with 1 to 12 carbon atoms which is optionally substituted with one or more fluorines and in which optionally 1 or 2 non-adjacent methylene units (CH2) can be replaced by oxygen, COO, OOC, CO and/or CHxe2x95x90CH.
The term xe2x80x9c1,2-, 1,3- or 1,4-phenylene, which is unsubstituted or substituted with one or more of the groups selected from halogen, CN and/or NO2 and in which 1 or 2 CH groups can be replaced by nitrogenxe2x80x9d includes, and is not limited to, in the present application 1,2-phenylene, 1,3-phenylene or 1,4-phenylene, 2-fluoro-1,4-phenylene, 2,3-difluoro-1,4-phenylene, 2,6-difluoro-1,4-phenylene, 2-chloro-1,4-phenylene, 2,3-dichloro-1,4-phenylene, 2,6-dichloro-1,4-phenylene, 2-cyano-1,4-phenylene, 2,3-dicyano-1,4-phenylene, 2-nitro-1,4-phenylene, 2,3-dinitro-1,4-phenylene, 2-bromo-1,4-phenylene, 2-methyl-1,4-phenylene, and pyridine-2,5-diyl, pyrimidine-2,5-diyl, and the like. Particular examples that are preferred include 1,4-phenylene, 2-fluoro-1,4-phenylene, 2,3-difluoro-1,4-phenylene, 2,6-difluoro-1,4-phenylene, pyridine-2,5-diyl, pyrimidine-2,5-diyl.
The term xe2x80x9c1,4- or 2,6-naphthalene, in which 1 or 2 CH groups can be replaced by nitrogenxe2x80x9d includes, and is not limited to, in the present application 1,4- or 2,6-naphthalene, 1-benzazine-2,6-diyl and 2-benzazine-1,4-diyl.
Examples of the term xe2x80x9calkyl with 1 to 12 carbon atoms which is optionally substituted with one or more fluorines and in which optionally 1 or 2 non-adjacent methylene units (CH2) can be replaced by oxygen, COO, OOC, CO and/or CHxe2x95x90CHxe2x80x9d include in the present application straight-chain and branched (optionally chiral) residues such as alkyl, alkenyl, alkoxy, alkenyloxy, alkoxyalkyl, alkoxyalkenyl, 1-fluoroalkyl, terminal trifluoromethylalkyl, terminal difluoromethylalkyl, terminal trifluoromethylalkoxy, and the like with 1 or, 2 to 16 carbon atoms. Examples of preferred residues are methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, 1-methylpropyl, 1-methylheptyl, 2-methylbutyl, 3-methyl pentyl, vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-hexenyl, 4-pentenyl, 4Z-hexenyl, 5-hexenyl, 6-heptenyl, 7-octenyl, methoxy, ethoxy, propyloxy, butyloxy, pentyloxy, hexyloxy, octyloxy, 1-methylpropyloxy, 1-methylheptyloxy, 2-methylbutyloxy, 1-fluoropropyl, 2-fluoropropyl, 2,2-difluoropropyl, 3-fluoropropyl, 3,3-difluoropropyl, 3,3,3-trifluoropropyl and the like. Especially preferred residues possess 1 or, respectively, 2 to 6 carbon atoms.
The term xe2x80x9chalogenxe2x80x9d may represent in the present application fluorine, chlorine, bromine and iodine, but especially fluorine and chlorine.
The structural and other preferences are expressed below on the basis of inter alia one or more of the following propertiesxe2x80x94good processability, thermal, chemical and electrochemical stability, ability to isomerise and/or dimerise, monodispersity and ready synthesis from commercially available starting materials.
Overall preferred structures for the groups PG1-3 are those listed below: 
wherein
S1 indicates the position of the spacer group
A1 may be 1,4-phenylene, which is unsubstituted or substituted with one or more of the groups selected from halogen, CN, NO2;
or pyrimidine-2,5-diyl, pyridine-2,5-diyl, 2,5-thiophenediyl, 2,5-furanylene, 1,4- or 2,6-naphthalene;
A2 may be 1,4-phenylene, which is unsubstituted or substituted from one and up to and including all available substitution positions with one or more of the groups selected from halogen, CN, NO2;
or pyrimidine-2,5-diyl, pyridine-2,5-diyl, 2,5-thiophenediyl, 2,5-furanylene, trans-1,3-dioxane-2,5-diyl or 1,4-piperidyl;
Z1 may be a direct bond, CH2CH2, COO, OOC, O(CH2)3, OCH2, CH2O, (CH2)3O, or (CH2)4;
n1 may be 0 or 1;
W, Y may each independently be H, halogen, CN, alkyl with 1 to 12 carbon atoms which is optionally substituted with one or more fluorines;
R1 may be H, halogen, CN, alkyl with 1 to 12 carbon atoms which is optionally substituted with one or more fluorines and in which optionally 1 or 2 non-adjacent methylene units (CH2) can be replaced by oxygen, COO, OOC and/or a CH2CH2 group may be replaced by CHxe2x95x90CH;
or even more preferred structures for the groups PG1-3 are given by the formula IIIA 
wherein
A4 may be 1,4-phenylene, which is unsubstituted or substituted from one and up to and including all available substitution positions with one or more of the groups selected from halogen, CN, NO2; or pyrimidine-2,5-diyl, pyridine-2,5-diyl, 2,5-thiophenediyl, 2,5-furanylene, 1,4- or 2,6-naphthalene;
Z3 may be a direct bond, CH2CH2, COO, OOC, O(CH2)3, OCH2, CH2O, (CH2)3O, or (CH2)4;
n3 may be 0 or 1;
A6 may be 
R2 may be alkyl with 1 to 12 carbon atoms which is optionally substituted with one or more fluorines and in which optionally 1 or 2 non-adjacent methylene units (CH2) can be replaced by oxygen, COO, OOC and CH2CH2 may be replaced by CHxe2x95x90CH.
According to an aspect of this invention a method of providing an alignment layer on a surface of a liquid crystal cell wall includes the step of depositing a layer of a material comprising at least one compound of Formula I on the surface, followed by exposure to actinic radiation, and controlling the exposure time and/or intensity of radiation used to provide a selected value of pretilt in a liquid crystal placed in contact with the exposed layer.
The radiation includes light, with a wavelength of 250-450 nm. Preferably the radiation is light with a wavelength of 300-400 nm.
According to a further aspect of this invention a liquid crystal device comprises a layer of a liquid crystal material contained between two cell walls both carrying electrode structures and surface treated to provide an alignment layer for liquid crystal molecules;
characterised in that:
the surface treatment is a layer of material comprising a compound of Formula 1 which has been exposed to actinic radiation.
The radiation includes light, with a wavelength of 250-450 nm. Preferably the radiation is light with a wavelength of 300-400 nm.
Compounds of Formula I can be prepared by various routes from commercially available starting materials. Typically 1,2,3-trihydroxypropane (glycerol) can be esterified with xcfx89-halogenoalkanoic acids in the presence of N,N-dicyclohexylcarbodiimide and 4-(dimethylamino)pyridine and a polar solvent, such as N,N-dimethylformamide or dichloromethane. The resultant bromides can then be alkylated in a Williams ether synthesis with a photoisomerisable/dimerisable group, such as 6-hydroxycoumarin, 7-hydroxycoumarin or alkyl 4-hydroxycinnamates, in the presence of a base, such as potassium carbonate, and a polar solvent, such as cyclohexanone or ethyl-methylketone. The bromides can also be esterified with a photoisomerisable/dimerisable group, such as cinnamic acids, in the presence of DBU and a non polar solvent, such as toluene or benzene. Similarly 1,2,3-trihydroxypropane (glycerol) can be alkylated with xcfx89-halogenoalkanols protected, for example as the THP derivative, in the presence of base, such as potassium tert-butylate, and a polar solvent, such as 1,2-dimethoxyethane or ethylene glycol dimethyl ether. After deprotection the resultant alcohols can then be esterified with a photoisomerisable/dimerisable group, such as anthracene carboxylic acids or cinnamic acids, in the presence of N,N-dicyclohexylcarbodiimide and 4-(dimethylamino)pyridine and a polar solvent, such as N,N-dimethylformamide or dichloromethane. The alcohols can also be alkylated in a Mitsunobu reaction, with a photoisomerisable/dimerisable group, such as 6-hydroxycoumarin, 7-hydroxycoumarin or alkyl 4-hydroxycinnamates, in the presence of a dehydrating agent, such as diethyl azodicarboxylate and triphenyl phosphine, and a polar solvent, such as tetrahydrofuran or N,N-dimethylformamide.
The photocross-linkable groups, such as cinnamic acids, cinnamate esters, cinnamonitriles, styrenes, stilbenes, vinylnaphthalenes, vinylpyridines, maleimides, thymines, coumarins, anthracenes, are generally either commercially available or readily accessible, for example coumarin and cinnamate derivatives can be prepared according to literature methods, such as the Perkin, Pechmann, Wittig-Horner or Knoevenagel reactions. See J R Johnson, xe2x80x9cThe Perkin Reaction and Related Reactionsxe2x80x9d, Organic Reactions, 1, 210-65, 1942; Wittig Horner Reaction: L Lombardo and R J K Taylor, Synthesis, 131, 1978; Knoevenagel Reaction: G Jones, Organic Reactions, 15, 204 (1967).
In order to obtain alignment layers in regions selectively limited by area, a solution of the photoactive propane derivative can, for example, firstly be prepared and then spread out using a spin-coating apparatus on a carrier coated with an electrode, e.g., a glass plate coated with indium-tin oxide (ITO) such that homogeneous layers of thickness typically in the range 50-200 nm result. Preferably the layers are in the range 0.05-50 xcexcm. Subsequently or simultaneously, irradiation can be applied to the region to be isomerised and/or dimerised (cross-linked), e.g., with a mercury high pressure lamp, a xenon lamp or a UV laser utilising a polariser and optionally a mask for the formation of structures. The duration and irradiation depends on the capacity of the individual lamps and can vary from a few minutes to several hours. The cross-linking can, however, also be effected by irradiating the homogeneous layer using filters which, e.g., let through only radiation suitable for the cross-linking reaction. Photosensitisors, such as acetophenone or benzophenone may be added to shorten the illumination time required for cross-linking. Non-zero tilt angles (xcex8) may be induced by illumination with plane polarised light from a non-perpendicular angle to the plane of the substrate.
The materials of the current invention when used in alignment layers are believed to be of particular benefit due to their lack of polydispersity.